(1) Field of the Invention
The present invention relates generally to reception of underwater signals in a multipath environment and, more specifically, to a variable integration apparatus and method for improving detection performance.
(2) Description of the Prior Art
Underwater acoustic sonar signals and other similar types of signals that propagate through fluids are more difficult to detect when degraded by a multipath environment. Multipath is a term that describes a type of environment wherein a transmitted signal follows more than one propagation path before returning to the receiver. As a result of the typically different and unequal signal propagation path lengths that the transmitted signal follows in the multipath environment, several delayed versions of the transmitted signal arrive at the receiver with each delayed signal having a different amplitude and phase. When the received multipath signals overlap, the sum of all the signals creates a composite received signal that has random amplitude and phase characteristics. When this occurs, an underwater object is difficult to detect using a conventional signal processor, such as a quadrature receiver.
Quadrature receivers and operation thereof are described in the book Detection of Signals in Noise by A. D. Whalen, published by Academic Press, N.Y. 1971, which publication is hereby incorporated herein by reference. While other types of receivers may be used in accord with the present invention, a quadrature receiver is typically used for reception/processing of acoustic signals and therefore the present invention is discussed in terms thereof. The quadrature receiver performs best when the received signal is coherent, i.e., constant but with an unknown amplitude and phase. When the transmitted signal has been affected by multipath interference, it becomes incoherent and the performance of the quadrature receiver is degraded.
U.S. Pat. No. 5,321,668, issued Jun. 14, 1994, to R. E. Rouquette, discloses a method for determining the ranges between pairs of transceivers deployed underwater. The transceivers transmit acoustic pulses to and receive acoustic pulses from other of such transceivers according to a coordinated schedule of individual transmission times and reception windows stored in each transceiver. A central controller in communication with all the transceivers compiles the individual schedules, collects the transmit and reception time data, and computes the ranges therefrom. The method does not include repetitively parallel processing of a stored received signal while altering respective processing periods during each processing iteration.
U.S. Pat. No. 5,208,786, issued May 4, 1993, to Weinstein et al., discloses a system to separate unknown signals that have been combined together through unknown linear filters and for which observations at multiple sensors are made. In a two channel circuit with two inputs and two sensors, the reconstructed source signals are assumed to be decorrelated such that the cross-correlation between the signals is near zero, e.g., propeller noise and a sonar signal. However, return signals detected in a multipath environment are generated from the same source and may not be assumed to have a cross-correlation near zero.
U.S. Pat. No. 4,910,718, issued May 20, 1990, to M. Horn, discloses a system for locating a source of acoustic emissions wherein two multi-element transducers are employed. The elements are subjected to an impinging acoustic wave at different moments in time depending upon the angle of the wave. The outputs from the elements of each transducer are compared with a look-up table to determine the angle. Then, another look-up table is used to determine the X-Y intersection of impinging wave angles from both transducers which locates the source. The system is not designed to detect a signal embedded in multipath noise.
Consequently, there remains a need for a receiver that offers more accurate detection of signals reflected in a multipath environment. Those skilled in the art will appreciate the present invention that addresses the above and other problems.